One of the liquid dispensers of the above type is known from, for example, the Japanese Unexamined Patent Publication No. 217271 of 1990.
The above Unexamined Japanese Patent Publication discloses a liquid dispenser having connected thereto a syringe and an air source and air aspirator via a dispensing solenoid valve and which includes an accumulator and pressure-regulation reducing valve provided in this order between the dispensing solenoid valve and air source upstream of the solenoid valve, an accumulator and suctorial-negative pressure reducing valve provided in this order between the dispensing solenoid valve and air aspirator upstream of the solenoid valve, and a controller that supplies a dispense command to the dispensing solenoid valve for a predetermined time while supplying a pressure setting signal corresponding to at least the residual amount of a liquid in the syringe to each of the pressure-regulation reducing valve and suctorial-negative pressure reducing valve. An embodiment of this liquid dispenser in which each of the pressure-regulation reducing valve and suctorial-negative pressure reducing valve uses an electropneumatic regulator is disclosed along with its circuit diagram in the above Japanese Unexamined Patent Publication.
As stated in the Japanese Examined Patent Publication No. 50418 of 1994, the above “electropneumatic regulator” is adapted to detect, when regulating delivery pressure, the pressure by means of a pressure sensor included therein, compare the detected pressure with a set pressure level, supply a pneumatic pulse to a pilot chamber by controlling a solenoid valve included therein correspondingly to the result of the comparison, open or close an inlet or outlet valve connected to the pilot chamber and thus regulate the delivery pressure from a main valve to the set pressure level. The electropneumatic regulator per se is well known.
It should be noted however that in such a conventional technology, the negative pressure in the syringe just after the liquid dispenser is switched on was found to vary more largely than that in the syringe after a while from the switch-on.
Namely, the negative pressure applied to act on a liquid in the syringe in order to prevent dripping is not sufficiently stable just after the apparatus is turned on, so that in case the negative pressure is insufficient, the liquid will drip from the end of a nozzle communicating with the syringe and having an outlet formed in the tip thereof, while in case the negative pressure is excessive, atmosphere will be sucked into the apparatus through the nozzle end.